1. Field of the Invention
The present invention relates to a metal oxide material having electric conductivity and a process for preparation of the same. More particularly, the present invention relates to a metal oxide material having superconductive characteristics and a process for preparation of the same. The metal oxide material according to the present invention can be used in a variety of fields, such as various sensors, electronic elements, computers, medical devices, magnet, power-transmission lines, energy devices and voltage standard and the like.
2. Related Background Art
Since oxide superconductors containing copper, which have been sequentially found in recent years, have Tc (superconduction critical temperature) considerably higher than that of the conventional niobium type superconductors, applicability of the superconductor of the foregoing type has been examined in a multiplicity of fields.
Among the foregoing oxide superconductors containing copper, Y-type (YBa.sub.2 Cu.sub.3 O.sub.7) material has been found and its applicability has been examined.
As the Y-type materials, there are superconductors having different crystalline structures and composition ratios. Their compositions are different in the quantity of copper and that of oxygen and the Y-type materials respectively are expressed by YBa.sub.2 Cu.sub.4 O.sub.8 (124 structure) and Y.sub.2 Ba.sub.4 Cu.sub.7 O.sub.y (y is about 15, 247 structure). Since superconductors of the foregoing type composed of the same component elements have different characteristics such as the critical temperature (Tc), they are considerably affected by synthesizing conditions such as the partial pressure of oxygen and temperature. A figure showing the relationship between the factors, such as the mixture ratio and the temperature, and the composition of the material which can be easily prepared is disclosed in, for example, Japanese Journal of Applied Physics Vol. 29, No. 12, December, 1990, pp. 2720-2724. According to the foregoing document, the 124 structure is rather stably present than the 123 structure at about 750.degree. C. or lower in a case where the ratio of positive ion is Y:Ba:Cu=1:2:3 under condition of an extremely usual partial pressure of oxygen of 1 atmospheric pressure. This means that, if the reaction temperature is lowered by a means, such as particulating of the raw material, a single-phase 123 structure cannot easily be obtained. If a temperature distribution is present at the time of heat treatment, a single phase cannot easily be realized. Further, the Y-type material essentially wants oxygen, and oxygen is introduced/discharged at 500.degree. to 700.degree. C. accompanying structural phase transition. Accordingly, heat treatment is very generally performed in which the Y-type material is subjected to annealing process at the foregoing temperature to convert the phase to a phase containing a large quantity of oxygen in order to improve the superconductive characteristics, for example, in order to raise Tc. It can be considered that the 124 structure is further stabilized during the foregoing process. That is, the Y-type (YBa.sub.2 Cu.sub.3 O.sub.7) material formerly encounters a problem that it cannot easily be made the single phase.
Generally, if the material is not the single phase, if the material is material in which two types of superconductors are present while being mixed, the averaged material characteristics of the two phases determine the characteristics of the material due to the difference in the physical properties such as Tc of each phase and the superconductive critical current density (Jc) and the like. Therefore, a sharp transition to the superconductive state cannot take place or the essential superconductive characteristics of a sole superconductor cannot be exhibited sufficiently depending upon the situation. In electronics application field using a Josephson device and so forth, the device cannot be designed if the superconductive characteristics are not stable. As a result, the presence of a plurality of superconductive phases is a critical defect when a reliable device having excellent performance is inserted to be obtained.
The "single phase state" which particularly attracts attention in this embodiment is a state in which the grain boundaries of the superconductor are combined to each other while depositing no impurity phase in the grain boundaries of the foregoing Y-type material. The foregoing state does not mean a state where non-superconductors are dispersed in the crystalline particles for, so-called "flux pinning".
A consideration is made to realize power generation from, for example, a magnet, which utilizes one of the characteristics of the superconductive phenomenon, that is, no electric resistance, and which generates a high level magnetic field while using satisfactory small electric power consumption by flowing a large quantity of loss-less electric currents. In this case, if the "single phase state" is not realized particularly in an oxide superconductor due to deposition of impurities in the crystalline grain boundary, the quantity of the electric current is excessively restricted. Therefore, deterioration in the performance occurs.
If the deposited substance is chemically unstable, the superconductive characteristics of the material excessively deteriorate and causes aging to take place. The grain boundary of the Y-type material is easily weakened due to the impurity phase or the deposition of an amorphous phase. Occurrence of the foregoing problem has been discussed in Oyo Butsuri, Vol. 60, No. 5 (1991), pp 462 to 465.
As the copper oxide superconductor containing carbon and relating to the metal oxide material according to the present invention, material having a composition Sr.sub.0.9 Ba.sub.1.1 Cu.sub.1.1 O.sub.2.2 (CO.sub.3).sub.0.9 has been disclosed in Nikkei Superconduction, Apr. 13, 1992. According to this disclosure, the temperature at which the resistance is zero is about 26K. Therefore, there arises a problem in that liquid helium or a large-cost cooling device must be used at the time of using the foregoing material. Physica C Vol.191 (1992) pp.434 to 440 has disclosed a similar material having a composition Sr.sub.2 CuO.sub.2 (CO.sub.3). Since Sr.sub.2 CuO.sub.2 (CO.sub.3) has no superconductivity at low temperature and exhibits a high electric resistance, it cannot be used as the oxide superconductor as well as the superconductor.
It is usually preferable that the critical temperature Tc is about 30K or higher, more preferably 50K or higher. The reason for this is that various superconductor products each containing the superconductive material, the critical temperature Tc of which is 30K, can usually stably be operated only when it is cooled to about a 10K lower than the critical temperature. Therefore, the cooling method is limited excessively. That is, it is necessary for the superconductor to be cooled to 30K or lower by using liquid helium as the cooling substance and heat insulation using liquid nitrogen or vacuum. Therefore, a great facility must be used to shield heat from the room temperature portions. Even if a cryopump is used, a great heat shield using vacuum or a plurality of heat insulating materials must be used. Although it is preferable that the temperature is about 30K or higher, more preferably 50K or higher, the heat shield can considerably easily be established in the foregoing temperature region. As a result, the cooling system can be simplified and the superconductive state can be maintained stably.